This invention relates to communication systems having immunity to interferring signals and more particularly to a correlation type detector employed in the receiver of such communication systems.
It is desirable in communication systems employed in various applications, such as providing command signals to a satellite or other similar command systems, to provide immunity to interferring signals so that the commands are not lost or interrupted. The interferring signal immunity is enhanced by the type of detector employed in the receiver of the communication system.
A first type of detector that may be employed in this type of communication system is classified as an invariant matched filter or correlator. This type of correlator will not respond to signals S.sub.o but will respond to signals S.sub.i, where i=1,2 . . . n, where each signal S.sub.i is interpreted as a particular transmitted symbol. The properties required of S.sub.i are that they are sufficiently dissimilar so that an interferring signal will not be likely to change one signal S.sub.i and to another. Further signals S.sub.i must be a small class of signals compared to the class of signals represented by S.sub.o and the S.sub.o signals must represent a sufficiently large class. An implementation of this type of detector can be the same as the correlator of this invention wherein the coder is eliminated and replaced by a constant code state.
A second type of detector which may be employed in such communication systems is a time variant matched filter or correlator. Phase reversal coding and frequency hopping are special cases of time variant correlator and time variant matched filter, respectively. The major problems with this type of detector are reliable means of synchronization, rapid and reliable acquisition and reacquisition of synchronization, stable memory circuits, and high processing rates compared to information rates. This type of detector requires that both the transmitter and detector coder run at all times which increases the possibility of coder jump, thereby, reducing system reliability.